1. Field of the Invention
The present invention relates to optical switches, liquid crystal microprism arrays, and free-space optical interconnectors used in optical-communication switching systems based on the propagation of rays of light as communication signals.
2. Description of the Prior Art
Heretofore, optical-communication switching systems have been regarded as important ways for communication services. In addition to optical fiber transmission lines, the optical-communication switching system for switching optical-communication signals as it is can be of use to realize high-speed and widespread communication services such as of high-resolution image data transfer and multimegabite data transfer. For constituting a large scaled network of optical switches as a specific optical communication path in the optical-communication switching system, generally, the network is composed of a lot of small optical switches. These optical elements are connected with each other by a plurality of optical fiber links as in the case of crossbar switches used for switching electric signals.
FIG. 1 shows an 8.times.8 optical switch (a Clos type switch) which is formed as a parallel arrangement of optical switch groups, consisting of a plurality of small optical switches in the type of 2.times.3, 4.times.4, and 3.times.2 respectively. Therefore, the 8.times.8 parallel optical switch comprises groups of: input fibers 1; 2.times.3 optical matrix switches 2; optical fiber interconnections 5; 4.times.4 optical matrix switches 3; optical fiber interconnections 7; 3.times.2 optical matrix switches 4; and output fibers 6, being arranged in that order.
The group of 2.times.3 optical matrix switches 2 includes four switches paralleled with each other in the direction of propagating the incident rays of light through the input fibers 1. Each of the 2.times.3 optical matrix switches has two input terminals connected with the respective input fibers 1 and three output terminals connected with the respective optical fibers 5.
The group of 4.times.4 optical matrix switches includes three switches paralleled with each other. Each of the 4.times.4 optical matrix switches has four input terminals connected with the respective optical fibers 5 and four output terminals connected with the respective optical fibers 7. Furthermore, the group of 3.times.2 optical matrix switches includes four switches paralleled with each other. Each of the 3.times.2 optical matrix switches 4 has three input terminals connected with the respective optical fibers 7 and two output terminals connected with the respective output fibers 6.
As shown in the figure, therefore, the output terminals of the 2.times.3 optical matrix switches 2 are connected with the different input terminals of the 4.times.4 optical matrix switches 3 through the fibers 5 respectively, while the output terminals of the 4.times.4 optical matrix switches 3 are connected with the different input terminals of the 3.times.2 optical matrix switches 4 through the fibers 7 respectively. Thus the two groups of the optical fibers form the first and second interconnections 5 and 3 among the groups of the optical switches 2, 3, and 4.
In spite of comprising small sized optical switches, however, the conventional network cannot be reduced its scale because of its complex arrangement of the optical fibers for the interconnection between the optical switches.
For making the network on a small scale without reducing its communication abilities, there is an idea of interconnecting between the optical elements by passing the optical signals through free-space. This kind of the propagation has the advantages, for example: (i) optical data is transmitted at an extremely higher speed compared with the electric one; and (ii) a volume of the space for the interconnections is much smaller than that of the optical elements such as the optical fibers and optical waveguides.
One of the proposed system using the free-space optical interconnections is an array of two-dimensional semiconductor optical gate switches that includes an array of multiple quantum well (MQW) optical modulators and an array of optical detectors (A. L. Lentine & D. A. B. Miller: "Evolution of the SEED Technology: Bistable Logic Gates to Optoelectronics Smart Pixels", IEE J. QE., vol. 29, pp 655-669, 19, and Japanese Patent Application Laying-open No. 6-130431). In addition, another proposed system using the free-space optical interconnections is an array of two-dimensional optical gate switches composed of a combination of an optical detector array and a surface-emitting laser array (Y. Tashiro et al., "High speed response in optoelectronics gated thyristor", Jpn. J. Appl. Phys., vol. 26, p. L274, 1987). In these systems, the two-dimensional semiconductor optical gate switch arrays are arranged as multiple stages interconnected by a plurality of light-beams passing through the free-space.
Hereinafter, a general term "two-dimensional optical element array" includes meanings of a two-dimensional optical gate switch array, a two-dimensional optical source array, a two-dimensional optical fiber array, and so on.
To keep pace with the above communication demands, technology for interconnecting between the arrays of two-dimensional optical elements becomes more important for achieving the free-space optical interconnection. For connecting the elements, as shown in FIG. 1, a plurality of light-beams must be replaced and distributed so as to make the optical interconnections of the optical elements with each other.
FIG. 2 shows a conventional optical switch system as a combination of arrays of two-dimensional optical elements with free-space optical interconnections as disclosed in Japanese Patent Application Laying-open No. 6-130431. The system comprises a first optical interconnect circuit 19, an array of micro-lenses 21, an array of two-dimensional optical gate switches 13, an array of micro-lenses 22, an array of .lambda./4 plate 14, a polarization beam splitter 16, and a second optical interconnect circuit 26. These elements are arranged in that order. As shown in the figure, a plurality of light-beams 8 is introduced into the first optical interconnection circuit 19 while a plurality of output light-beams 18 is produced from the second interconnect circuit 26.
The interconnection circuit 19 comprises three birefringent plates 10-12 and a patterned 1/2 wavelength (.lambda./2) plate 20 between the plates 11 and 12. The polarization beam splitter 16 has a reflection surface 15 by which a direction of propagating the light-beam is changed when the beam counters. The second optical interconnect circuit 26 comprises a patterned .lambda./2 plate 24, a birefringent plate 17, and a .lambda./4 plate 25.
The optical interconnection can be performed by changing the direction of propagating the light-beam by taking advantage of splitting the light-beam into polarized components. If a circularly polarized light passes through the birefringent plate, it will be split into two polarized components: p- and s- polarized beams, resulting that these two components propagate toward the different directions respectively. Each patterned .lambda./2 plate 20 or 24 comprises .lambda./2 plate segments (i.e., hatched portions in the figure) corresponding to the p-polarized components and transparent plate segments corresponding to S-polarized components.
In the document of Lentine and Miller described above, the optical interconnection is performed by changing the direction of propagating the light-beams by means of a computer generated hologram. In the document of Tashiro et al., on the other hand, the optical interconnection is performed by means of an optical system consisting of a plurality of lenses for focusing and reconstructing the light-beams.
FIG. 3 shows a conventional optical switch system with a free-space optical interconnection in which optical elements are adjustably connected with each other as described in Japanese Patent Application Laying-open No. 60-247228.
In this system, a converging rod lens 27 has a liquid crystal wedge cell 29 on one side thereof and output optical fibers 31-34 on another side thereof. The liquid crystal wedge cell 29 comprises a liquid crystal sandwiched between two transparent electrodes 28 connecting with a power supply 30.
An incident angle of the input light can be varied in accordance with a reflective index of the liquid crystal 29 depending on a voltage placed between the two electrodes 28 by the power supply 30, resulting in that the optical fibers connected with the rod lens 27 are switched.
In the optical system described above, however, the transparent electrodes cover a whole surface of both sides of the liquid crystal 29 so that a plurality of the input beams cannot be deflected individually. Also, an increase in numbers of the output optical fibers requires an increase in the size of an area of the rod lens to be connected, which results in the rod lens of larger diameter and the liquid crystal with a larger bottom area. Therefore, a response speed of the liquid crystal can be decreased because of a higher voltage placed between the electrodes.
To solve these problems, as shown in FIG. 4, another type of optical switch system is disclosed in UK Patent Application No. G292184560. The optical switch comprises an input optical fiber 35 and an output optical fibers 36, 37; a converging rod lens 38; a polarizer 39; a glass base pate 40; and a liquid crystal 43 sandwiched between a transparent electrode 41 and a conductive reflector 42, which are arranged in that order. Furthermore, the transparent electrode 41 and the conductive reflector 42 are connected with a power supply 44 to apply a voltage on the liquid crystal 43.
An incident light-beam introduced by the optical fiber 35 is transmitted into the liquid crystal layer 43 through the rod lens 38. Then the light-beam is reflected by the conductive reflecting plate 42 to change its propagating direction toward the output optical fiber 36 or 37. The reflected beam is passed through the liquid crystal 43 and the rod lens 38 again and outputted from one of the fibers 36 and 37. In this case, the liquid crystal layer 43 acts as a prism for changing the direction of propagating the light-beam because a thickness of the liquid crystal cell is varied.
A deflection angle is depended on the refractive index of the liquid crystal cell, while the reflective index of the liquid crystal is depended on a voltage applied between the two electrodes by the power supply. Therefore the reflected light-beam can be selectively introduced into the optical fiber 36 or 37 by changing the voltage to be applied to the liquid crystal.
In spite of the above construction, however, a plurality of the input beams cannot be deflected individually because the transparent electrodes cover a whole surface of both sides of the liquid crystal.